Districts and teachers definitely vary. I've certainly read posts here where they've been accommodating in addition to the horror stories. The more asynchronous the child's development the harder it is to meet their needs in a group setting. Our district does a reasonable job at accelerating kids 1 or 2 years ahead in math especially as they get older. I'm very grateful for the walk to math options that my older son had starting in Kindergarten. However, if a kid is beyond that point I don't see much to do except wait until they are mature enough to advocate for some kind of independent study.